1. Field of the Invention
The present invention relates to a light emitting element, an array-shaped light source and a manufacturing method thereof, and an optical signal transmitter used in a light source for a lighting device, etc., or a communication device using optical fibers, etc.
2. Description of the Related Art
A parallel transmission system attracts public attention as a transmission system of data by progress of a recent optical communication technique. In this parallel transmission system, a large amount of information is not transmitted at a high speed by a single optical fiber, but data are transmitted in parallel by using many optical fibers.
In such a parallel transmission system, data can be divided in parallel and transmitted. Accordingly, when information is transmitted at the same information transmitting speed, a data transmitting speed per optical fiber can be reduced in comparison with a case in which data are transmitted by one optical fiber. Further, when data are transmitted by one optical fiber, the data are normally transmitted after processings such as parallel-serial conversion, coding of these data, etc. It is also necessary to perform decoding and serial-parallel conversion of received data. However, in the parallel transmission system, parallel data can be transmitted as they are. After these parallel data are received, these data can be used as they are. Therefore, the parallel transmission system can be simplified.
In such a parallel transmission system, it is effective to integrate light emitting elements as light sources in an array shape so as to simplify coupling of the light emitting elements and optical fibers and make the parallel transmission system compact, etc. It is considered to keep characteristics of such parallel transmission that a light emitting diode (LED) is used as a kind of light emitting element since the light emitting diode is easily treated and has high safety in comparison with a laser diode. The light emitting diode is used in fields of a small-scale LAN, data transmission within a device, HA within home, etc. in which the information transmission speed may be reduced in comparison with a trunk system line, etc.
It is advantageous to use an edge emitting LED as the light emitting diode in comparison with a surface emitting LED in coupling of the LED and an optical fiber. In the edge emitting LED, light emitting efficiency is generally high in comparison with the surface emitting LED and directivity of light in a direction perpendicular to a substrate can be strengthened by using a hetero structure, etc. Accordingly, future utilization and enlargement of the edge emitting LED are expected as a light emitting element for parallel transmission.
Light emitting power of the light emitting diode is small in comparison with the laser diode so that high coupling efficiency of the light emitting diode and an optical fiber is required.
When the light emitting diode is used as a light emitting element, a light emitting diode of a surface emitting type is generally proposed to increase coupling efficiency of the light emitting diode and an optical fiber. For example, this light emitting diode is shown in Japanese Patent Publication (KOKOKU) No. 5-58272. In FIG. 1 showing this light emitting diode, a spherical lens 202 is arranged on a light taking-out portion of a light emitting element 201.
Each of Japanese Patent Application Laying Open (KOKAI) Nos. 63-132487 and 63-271981 shows a technique for improving the coupling efficiency of the light emitting diode and an optical fiber by processing a light emitting facet of the light emitting diode of an edge emitting type with predetermined curvature so that the light emitting diode functions as a cylindrical lens or a spherical lens.
FIG. 2 is a view showing one example of the light emitting diode shown in Japanese Patent Application Laying Open (KOKAI) No. 63-132487. In this light emitting diode, a light emitting face 206 is formed in a semi-circular shape by a photolithographic process and etching so that the light emitting diode functions as a cylindrical lens on the light emitting face 206. Light 207 emitted from the light emitting face 206 is converged at an end of an optical fiber so as to increase the coupling efficiency of the light emitting diode and the optical fiber.
Each of FIGS. 3a and 3b is a view showing one example of the light emitting element shown in Japanese Patent Application Laying Open (KOKAI) No. 63-271981. In the light emitting diode shown in FIG. 3a, a light emitting facet of this diode is formed by a photolithographic process and dry etching and a projecting portion 208 is formed with predetermined curvature so that the light emitting diode functions as a cylindrical lens. In the light emitting diode shown in FIG. 3b, a projecting portion 209 having a spherical shape is formed on a light emitting facet of this diode so that the light emitting diode functions as a spherical lens.
However, in the light emitting diode as shown in Japanese Patent Application Laying Open (KOKAI) No. 5-58272, the lens 202 must be arranged as a part separated from the light emitting element 201 so that the number of parts is increased. Further, an assembling process of this lens 202 is required. Therefore, manufacturing cost of the light emitting diode is increased and a process for manufacturing the light emitting diode is complicated.
In the examples shown in Japanese Patent Application Laying Open (KOKAI) Nos. 63-132487 and 63-271981, the light emitting facet is processed in the shape of a curved surface or a spherical shape so that the light emitting diode functions as a lens. In this case, light emission must be restrained approximately on the light emitting facet of the curved surface shape providing lens effects. Namely, as shown in FIG. 4a, when a light emitting region is formed at a portion 208 near the light emitting facet of the curved surface shape providing the lens effects, a light component of diffuse light spontaneous, emitting light 209 near the light emitting facet of the curved surface shape is emitted from an entire region of the light emitting facet of the curved surface shape so that the lens effects are greatly reduced. Accordingly, to effectively show the lens effects, it is necessary to emit light only in a limited light emitting region 211 as shown in FIG. 4b by restraining light emission in a portion 210 near the light emitting facet of the curved surface shape providing the lens effects without forming this near portion 210 in a light emitting region.
In accordance with the construction shown in FIG. 4b, light emitted in the limited light emitting region 211 is guided to the portion 210 near the light emitting facet of the curved surface shape in which light emission is restrained. This light is then converged as lens effects in this portion 210 so that a desirable shape of the emitted light can be obtained. However, in this case, the portion 210 near the light emitting facet of the curved surface shape restraining the light emission becomes an absorption layer with respect to light emitted in the light emitting region 211 so that an amount (emitting efficiency) of light emitted from this light emitting element is reduced. Namely, in the case of FIG. 4b, coupling efficiency of the light emitting element and an optical fiber can be improved, but light emitting efficiency is reduced. Therefore, an electric current injected into the light emitting element must be increased to maintain the light emitting efficiency so that power consumption of a signal transmitting unit is increased and the light emitting element is easily degraded.